Various techniques for restraining the rider of a motorcycle with an airbag system mounted to the motorcycle are known. For example, known techniques include a technique of restraining the rider of a motorcycle in the event of a head-on collision by inflating an airbag housed in a case mounted to the body frame (refer to Japanese Unexamined Patent Application Publication No. 2002-137777). The technique presents the possibility of providing a wide restraint area of the airbag. However, for an airbag system to be mounted to a vehicle in which the periphery of the rider is open, such as a motorcycle, there is a great demand for improving the performance of restraining the rider by inflating the airbag in a desired state in the event of a head-on collision.